1. Field of the Invention
The present invention relates to a method of producing an electrolytic capacitor using a valve metal such as aluminum, tantalum or the like and, more particularly, to a method of producing an electrolytic capacitor using a conductive polymer as a solid electrolyte.
2. Description of the Related Art
As a method of producing a conductive polymer to be turned into a solid electrolyte of an electrolytic capacitor by chemical oxidation polymerization, for example, a one-fluid method has hitherto been used. The one-fluid method introduces a mixed solution containing of a monomer to be turned into a conductive polymer by polymerization (hereinafter abbreviated to a "monomer solution") and an oxidizing agent capable of polymerizing the monomer by oxidation (hereinafter abbreviated to an "oxidizing agent solution") on a surface oxide layer of a porous valve metal to be turned into a capacitor, thereby to form a conductive polymer.
In the one-fluid method, however, the reaction is initiated immediately after the monomer and oxidizing agent were mixed and, in the case of a porous body having complicated pores such as tantalum sintered body, the reaction solution is deactivated before arriving the deep portion of pores and a conductive polymer is not formed at the center portion of a sintered body. As disclosed in Japanese Laid-open Patent Publication No. 6-310380, there is a method of introducing a mixed solution of a monomer and an oxidizing agent on an oxide layer of a porous body at low temperature and raising the temperature of the porous anode or mixed solution, thereby to make the amount of a conductive polymer formed at the center portion of the porous anode equal to the amount formed at the periphery of the surface. In the one-fluid method, however, since the polymerization reaction can not be completely terminated even when mixing the monomer solution with the oxidizing agent solution at low temperature, consumption of the monomer and oxidizing agent becomes larger.
Even when the monomer solution and oxidizing agent solution can be cooled to an ultra-low temperature and the polymerization reaction can be drastically inhibited, the cost of the production unit and running cost increase, which is uneconomical.
Accordingly, as disclosed in U.S. Pat. No. 4,697,001, there is generally used a two-fluid method of alternatively dipping a porous body to be turned into a capacitor in a monomer solution and an oxidizing agent solution without mixing the monomer solution and the oxidizing agent solution, that is, bringing the porous anode whose pores are impregnated with one of the monomer solution and oxidizing agent solution into contact with the other, thereby to form a conductive polymer in the inner spaces of the pores by polymerization.
However, when the porous anode impregnated with one solution among two solutions of the monomer solution and oxidizing agent solution is dipped in the other solution, the solution, with which the porous anode was impregnated, momentarily diffuses into the other solution on the external surface of the porous anode. Therefore, the conductive polymer is not easily adhered and formed on the external surface of the porous anode in comparison with the inner spaces of a lot of pores of the porous anode. When the external surface of the porous anode is not completely coated with the conductive polymer, a dielectric layer on the external surface is deteriorated by stress, resulting in large leak current. Therefore, a conductive polymer forming step must be repeated many times so as to obtain a capacitor with complete intrinsic capacity and small leak current by completely coating the dielectric layer formed on the internal surfaces of a lot of the pores and the external surface of the porous anode with the conductive polymer.